In order for immune cells, such as T cells, to respond to foreign proteins, two signals must be provided by antigen-presenting cells (APCs) to resting T lymphocytes (Jenkins, M. and Schwartz, R. (1987) J. Exp. Med. 165:302-319; Mueller, D. L. et al. (1990) J. Immunol. 144:3701-3709). The first signal, which confers specificity to the immune response, is transduced via the T cell receptor (TCR) following recognition of foreign antigenic peptide presented in the context of the major histocompatibility complex (MHC). The second signal, termed costimulation, induces T cells to proliferate and become functional (Lenschow et al. (1996) Annu. Rev. Immunol. 14:233). Costimulation is neither antigen-specific, nor MHC restricted and is thought to be provided by one or more distinct cell surface polypeptides expressed by APCs (Jenkins, M. K. et al. (1988) J. Immunol. 140:3324-3330; Linsley, P. S. et al. (1991) J. Exp. Med. 173:721-730; Gimmi, C. D., et al. 1991 Proc. Natl. Acad. Sci. USA 88:6575-6579; Young. J. W. et al. (1992) J. Clin. Invest. 90:229-237; Koulova, L. et al. (1991) J. Exp. Med. 173:759-762; Reiser, H. et al. (1992) Proc. Natl. Acad. Sci. USA 89:271-275; van-Seventer, G. A. et al. (1990) J. Immunol. 144:4579-4586; LaSalle, J. M. et al. (1991) J. Immunol. 147:774-80; Dustin, M. I. et al. (1989) J. Exp. Med. 169:503; Armitage. R. J. et al. (1992) Nature 357:80-82: Liu, Y. et al. (1992) J. Exp. Med. 175:437-445).
Such immune cells have receptors that transmit immunomodulatory (e.g., costimautory and coinhibitory) signals. For example, T cells have T cell receptors and the CD3 complex, B cells have B cell receptors, and myeloid cells have Fc receptors. In addition, immune cells bear receptors that transmit signals that provide costimulatory signals, or receptors that transmit signals that inhibit receptor-mediated signaling. For example, CD28 transmits a costimulatory signal to T cells. After ligation of the T cell receptor, ligation of CD28 results in a costimulatory signal characterized by, e.g., upregulation of IL-2rα, IL-2rβ, and IL-2rγ receptor, increased transcription of IL-2 messenger RNA, and increased expression of cytokine genes (including IL-2, IFN-γ, GM-CSF, and TNF-α). Transmission of a costimulatory signal allows the cell to progress through the cell cycle and, thus, increases T cell proliferation (Greenfield et al. (1998) Crit. Rev. Immunol. 18:389). Binding of a receptor on a T cell which transmits a costimulatory signal to the cell (e.g., ligation of a costimulatory receptor that leads to cytokine secretion and/or proliferation of the T cell) by a B7 family molecule, such as B7-1 or B7-2, results in costimulation. Thus, inhibition of an interaction between a B7 family molecule, such as B7-1 or B7-2, and a receptor that transmits a costimulatory signal on an immune cell results in a downmodulation of the immune response, specific unresponsiveness, termed immune cell anergy, clonal deletion, and/or exhaustion. Inhibition of this interaction can be accomplished using, e.g., anti-CD28 Fab fragments, antibodies to B7-1 or B7-2, or by using a soluble form of a receptor to which a B7 family member molecule can bind as a competitive inhibitor (e.g., CTLA41g).
Inhibitory receptors that bind to costimulatory molecules have also been identified on immune cells. Activation of CTLA4, for example, transmits a negative signal to aT cell. Engagement of CTLA4 inhibits IL-2 production and can induce cell cycle arrest (Krummel and Allison (1996) J. Exp. Med. 183:2533). In addition, mice that lack CTLA4 develop lymphoproliferative disease (Tivol, et al. (1995) Immunity 3:541; Waterhouse et al. (1995) Science 270:985). The blockade of CTLA4 with antibodies can block an inhibitory signal, whereas aggregation of CTLA4 with antibody transmits an inhibitory signal. Therefore, depending upon the receptor to which a costimulatory molecule binds (i.e., a costimulatory receptor such as CD28 or an 30 inhibitory receptor such as CTLA4), certain B7 molecules including B7-4 can promote T cell costimulation or inhibition.
PD-1 is a member of the immunoglobulin family of molecules (Ishida et al. (1992) EMBO J. 11:3887; Shinohara et al. (1994) Genomics 23:704). PD-1 was previously identified using a subtraction cloning based approach designed to identify modulators of programmed cell death (Ishida et al. (1992) EMBO J. 11:3887-95; 5 Woronicz et al. (1995) Curr. Top. Microbial. Immunol. 200:137). PD-1 is believed to play a role in lymphocyte survival, e.g., during clonal selection (Honjo (1992) Science 258:591; Agata et al. (1996) Int. Immunology. 8:765; Nishimura et al. (1996) Int. Immunology 8:773). PD-1 was also implicated as a regulator of B cell responses (Nishimura (1998) Int. Immunology 10:1563). Unlike CTLA4, which is found only on T cells, PD-1 is also found on B cells and myeloid cells. The previous discovery that PD-1 binds to PD-1 ligands, such as PD-L1 and PD-L2, placed PD-1 in a family of inhibitory receptors with CTLA4. While engagement of a costimulatory receptor results in a costimulatory signal in an immune cell, engagement of an inhibitory receptor, e.g., CTLA4 or PD-1 (for example by crosslinking or by 15 aggregation), leads to the transmission of an inhibitory signal in an immune cell, resulting in downmodulation of immune cell responses and/or in immune cell anergy. While transmission of an inhibitory signal leads to downmodulation in immune cell responses (and a resulting downmodulation in the overall immune response), the prevention of an inhibitory signal in cells, such as immune cells leads, to upmodulation of immune cell responses (and a resulting upmodulation of an immune response).
It is currently unknown whether coinhibitory receptors and ligands exist, especially because such well-known molecules involved in mediating coinhibitory signals and resulting downregulation of immune responses (e.g., CTLA4, PD-1, PD-L1, PD-L2, etc.) do not account for the full spectrum of observed immunoinhibitory responses. Accordingly, there is a need to identify additional coinhibitory receptors and/or ligands that play physiologically important roles in regulating immune responses.